1. Field of the Invention
The present invention relates to an antenna coil mounted in an RFID wireless tag, and to an antenna device including the antenna coil.
2. Description of the Related Art
RFID (Radio Frequency Identification Device) systems for performing contactless communication between devices using electromagnetic field signals have been used in various applications. In RFID systems, a magnetic flux generated by causing a current to flow in a transmitter coil produces a change in a magnetic field of a receiver coil to enable the exchange of information between the transmitter coil and the receiver coil.
For example, a system has been developed in which a user holds a mobile phone terminal equipped with an RFID wireless tag, instead of a commuter pass, over an automatic ticket gate of a station to perform communication to exchange user information before the user passes through the gate. In this case, it is easy to supply a current to a coil provided in the automatic ticket gate. Thus, a method in which a current is supplied to the coil provided in the automatic ticket gate to generate a magnetic flux and in which the magnetic flux is caused to pass through a coil axis of a coil of the mobile phone terminal is used. That is, the coil of the automatic ticket gate is a transmitter-side coil, and the coil of the mobile phone terminal is a receiver-side coil.
Another system has been developed in which a user holds a mobile phone terminal over a poster to perform communication to store information contained in the poster in the mobile phone terminal. In this case, it is difficult to supply a current to a coil provided in the poster. Thus, a method in which a current is supplied to a coil of the mobile phone terminal to generate a magnetic flux and in which the magnetic flux is caused to pass through a coil axis of the coil provided in the poster is used. That is, the coil of the mobile phone terminal is a transmitter-side coil, and the coil of the poster is a receiver-side coil.
With the increased number of methods for using RFID systems by using mobile phone terminals, there is an increasing demand for operating antenna coils mounted in the mobile phone terminals not only as receiver coils but also as transmitter coils.
An antenna coil described in PCT Japanese Translation Patent Publication No. 2001-517406 includes a transmitter coil and a receiver coil, which are provided on a single magnetic core. FIG. 11 is a perspective view showing the antenna coil described in PCT Japanese Translation Patent Publication No. 2001-517406. An antenna coil 900 includes a transmitter coil having a first transmitter coil portion 909a and a second transmitter coil portion 909b, and a receiver coil 906 located between the first transmitter coil portion 909a and the second transmitter coil portion 909b so that the transmitter coil and the receiver coil 906 are wound around a magnetic core 902.
When the antenna coil 900 described in PCT Japanese Translation Patent Publication No. 2001-517406 is used in an RFID system which utilizes a mobile phone, the antenna coil 900 is mounted such that a principal surface of the antenna coil 900 and a principal surface of the mobile phone terminal are parallel to each other. A user of the mobile phone terminal usually holds the principal surface of the mobile phone terminal over an external device, and it is necessary to provide superior communication at the principal surface of the antenna coil 900. However, with this use, the antenna coil 900 may not provide superior communication. The reasons therefor will be described below.
In the antenna coil 900, the transmitter coil portions 909a and 909b, and the receiver coil 906 have different directional characteristics. The transmitter coil portions 909a and 909b have opposite winding directions with respect to the coil axis, and each has a directional characteristic of increasing transmission intensity with respect to the direction of the principal surface of the antenna coil 900. The receiver coil 906, on the other hand, is a single coil, and has a directional characteristic of increasing sensitivity with respect to the direction of the coil axis of the antenna coil 900. When using a mobile phone terminal having an RFID transmission and reception function, a user may not change the orientation of the mobile phone terminal between transmission and reception. Therefore, depending on the orientation of the mobile phone terminal, the communication state greatly differs between transmission and reception.